In underground mining and drilling operations, it is often desired to increase the permeability and hence the productivity of a formation which has been penetrated by a well. Reasons for an unacceptably low rate of flow from a well may include such natural factors as the low permeability of the material which comprises the formation or the absence of fissures or other natural channels which have the effect of increasing the effective area exposed for production. Even after a well has successfully penetrated a formation and oil or any other product is being produced at an acceptable rate, continuous operation of the well over a period of time may result in the accumulation of asphaltic and waxy compounds or other materials which tend to clog the well and reduce its productivity.
An effective way in many cases to remedy one or more of these problems is by detonating a liquid explosive within the well bore or the formation. A number of techniques for explosive fracturing are described, for example, U.S. Pat. No. 3,191,678 to Hinson. Several techniques have been employed to detonate explosives in well bores, including timed detonators, detonators controllable through a wire line from the surface, and contact-actuated detonators. Contact-actuated detonators may be set-off electrically by an internal power supply through contact with a packer or similar device set in the well bore at the desired level (Hinson), or may be detonated by a "go-devil" dropped from the surface (e.g. U.S. Pat. No. 2,254,979), or may be inertially actuated when brought suddenly to a stop against a device set in the well bore (e.g. U.S. Pat. No. 3,410,214).
Inertially actuated detonators can be very hazardous for certain applications. Generally, a fluid column in the well limits their rate of travel down the casing and hence requires that they be set to go off when subject to a relatively small inertial force. Yet because of abrupt pressure changes often encounted in pumping fluids into a formation, the detonators may be subject to inertial forces adequate to be set off before ever reaching the pre-set depth stop.
Previously known contact-actuated detonators have also posed safety problems, such as the possibility of being detonated by sudden jarring due to pressure changes in the well while they are being loaded down to the desired depth. Some have also required alignment within narrow tolerances between special contact probes and the detonator contact, so that any misalignment readily leads to detonation failure, in itself a dangerous situation.